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They are functionally identical when mere referencing an array key name, but if you were to use a variable:

$array['$key'] : Would pass a literal string '$key'
$array["$key"] : Would pass the value of $key

If you were to use:

$array[key]

Then PHP will first search for a defined constant named "key", if it doesnt find this, then it will regard it as a key name. If you were to set your error reporting to E_ALL you would see the results of using arrays in this fashion, and its to be avoided as it results in a loss of performance.

Similar could be said for $array["$key"], as PHP will expect a string, but will also parse that string for variables. Another performance hit. Just use $array[ $key ].

THE INSTRUCTIONS BELOW ARE OLD AND MAY BE INACCURATE.
THIS INSTALL METHOD IS NOT RECOMMENDED, IT MAY RUN
OVER YOUR DOG. <-- MediaWiki installation guide